I'm guessing this has to do the recent removal of bison/flex/etc from the system profile: build.log and emerge --info to follow.
Created attachment 281303 [details] emerge --info
Created attachment 281305 [details] build.log.gz
Manually emerging flex and bison works around the issue.. I'm not sure if it was flex or bison (or both) that was needed.
I've added a dependency in the mysql/mysql-v2 eclasses on virtual/yacc. Feel free to reopen if you still hit this issue.